The time-dependent stress evolution, the result
ing residual
stresses as well as the microstructure of the heat treatable low alloyed steel AISI 4140
induced by laser surface spot harden
ing was
investigated systematically by means of synchrotron X-ray diffraction. In‐
situ stress analyses with a time resolution up to 100 ms were carried out at the synchrotron beaml
ines P05@PETRAIII(*), DESY, Hamburg and PDIFF@ANKA, Karlsruhe, by the application of the measurement and evaluation approach for very fast X-ray diffraction stress analyses. Dur
ing the laser surface spot harden
ing us
ing a homogenization work
ing optic with a spot size of approx. 8 × 8 mm
2 at a maximum temperature T
A,max of 1150 °C and heat
ing/cool
ing rates v
heat/cool of 1000 K/s time-resolved diffraction data were collected for various measurement positions
inside and outside of the processed zone aim
ing to analyze the different orig
ins for residual stress build-up. The
in-
situ tests were supplemented by X-ray residual stress analyses and microscopical
investigations of the microstructure subsequent to the laser harden
ing process (ex-
situ analyses).
The results show that inside of the martensitic transformed region (process zone) in radial and in tangential direction homogeneous compressive residual stresses are generated. The data of the in-situ diffraction experiments reveal that these compressive residual stresses develop due to (i) local compressive elasto-plastic deformations and (ii) local phase specific transformation strains. Outside the process zone, the compressive residual stresses are balanced by rather high inhomogeneous tensile residual stresses. By means of the in-situ determined diffraction data it is proven that these tensile residual stresses have their origin in the superposition of (i) quenching effects outside the process zone, (ii) local elasto-plastic deformations and (iii) the effect of phase transformations in the nearby process zone.